Leaf seals may be used to form a seal between two relatively rotating components in order to maintain a relatively high pressure on one side of the seal and a relatively low pressure on the other. A leaf seal is arranged with a large number of typically rectangular leaves which are held at a defined angle to the radial all the way round the seal circumference. The leaves give the seal a low stiffness, and are packed together such that the total leakage through the seal is reduced. Nonetheless, interleaf gaps do provide the seal with a porous aerodynamic working section. Such seals may be used, for example, in gas turbine engines.
FIG. 1 shows schematically a cut-away perspective view of a portion of a leaf seal 31 comprising a pack of leaves 32. FIG. 2 shows (a) a view along the axial direction of an arc segment of the pack to better show some of the leaves 32 edge-on, and (b) a plan view of a single leaf 32.
The leaves 32 each have a root portion 40 and a working portion 41, and have a width w in the axial direction and a thickness t. The leaves alternate with spacer elements 33 at their root portions 40, and are secured thereat to a backing ring 34 of a housing, which typically also comprises front 35a (high pressure side) and rear (low pressure side) 35b rigid coverplates. The working portions 41 of the leaves 32 present end edges 36 towards a surface 37 of a rotating component (shaft) generally rotating in the direction depicted by arrowhead 38. The leaves 32, and in particular the end edges 36 of the leaves 32, act against the surface 37 in order to create a seal across the assembly 31. Each leaf 32 is sufficiently compliant in order to adjust with rotation of the surface 37, so that a good sealing effect is created. The spacers 33 ensure that flexibility is available to appropriately present the leaves 32 towards the surface 37 which, as illustrated, is generally with an inclined angle between them. The spacers 33 also help to form interleaf gaps 39 between adjacent working portions 41 of the leaves 32. A leakage flow through these gaps 39 is induced by the pressure differential across the seal.
In a conventional leaf seal, such as that shown in FIGS. 1 and 2, the pressure drops from a high pressure on one side of the pack to a low pressure on the other side of the pack. In some circumstances, this pressure drop can contribute to leaf blow-down in which the end edges 36 bear down strongly on the surface 37. Although a limited amount of blow-down is desirable to create a good seal between the end edges 36 and the surface 37, excessive blow-down deflection causes high end edge contact loading and wear. Excessive blow-down deflection can also lead to increased wear of the rotor. The wear of the end edges and/or the rotor can limit the usable life of the seal.